Mental Hallucinations

The mind informing the senses, instead of the senses informing the
mind. There can be no doubt that the senses may be excited by the mind
(from within, as well as from without). Macbeth saw the dagger of his
imagination as distinctly as the dagger which he held in his hand.
Malebranche declared that he heard the voice of God. Descartes thought
he was followed by an invisible person, telling him to pursue his
search for truth. Goethe says that, on one occasion, he met an exact
counterpart of himself. Sir Walter Scott was fully persuaded that he
had seen the ghost of the deceased Byron. All such hallucinations (due
to mental disturbances) are of such stuff as dreams are made of.